1. Field of the Invention
The present invention relates to a heat treatment equipment provided with a heating chamber having a plurality of regenerative combustors each provided with a direct-fired burner having a porous heat accumulating body built therein and applying a heat treatment to an object in the heating chamber; an installing method of a porous heat accumulating body when applying a heat treatment to the object in the heating chamber having a plurality of such regenerative combustors; and a manufacturing method of a heat-treated object using such a heat treatment equipment; a heat treatment equipment provided with a space in which a plurality of heat exchangers each having a built-in porous heat accumulating body are attached; an installing method of a porous heat accumulating body in such a heat treatment equipment; and a manufacturing method of a heat-treated object, using such a heat treatment equipment. More particularly, the invention relates to a heat treatment equipment, an installing method of a porous heat accumulating body, and a manufacturing method of a heat-treated object, in which the substantial average surface pore diameters of the porous heat accumulating bodies are not uniform for the plurality of direct-fired burners or the plurality of regenerative combustors, or the plurality of heat exchangers. The invention relates also to a selecting method of a porous heat accumulating body for achieving uniform substantial average surface pore diameters of the porous heat accumulating bodies in the plurality of direct-fired burners, or the plurality of regenerative combustors, or the plurality of heat exchangers. Furthermore, the invention relates to a component member of a used porous heat accumulating body used so that the substantial average surface pore diameters of the porous heat accumulating bodies are not uniform for the plurality of direct-fired burners, or the plurality of regenerator combustors, or the plurality of heat exchangers.
2. Description of the Related Art
Unless otherwise discriminately referred to, a pore and a pseudo-pore (see the definition described later) will be generically referred to as “pores”. A heat exchanger having a built-in porous heat accumulating body will be referred to as a “regenerative heat exchanger” (or simply referred to as a “heat exchanger”, depending upon context) for convenience's sake so far as a gas flows through this heat accumulating body, irrespective of whether or not the heat accumulator is directly used for heat exchange of the heat exchanger.
1. Choking of Porous Heat Accumulating Body
There is well known a heat treatment equipment which is provided with a heating chamber having a plurality of regenerative combustors or regenerative heat exchangers each having a regenerative burner having a built-in porous heat accumulating body, and applies a heat treatment to an object in this heating chamber.
During operation of a heat treatment equipment of this kind, the porous heat accumulating body carries out heat exchange through alternate passage of a low-temperature gas and a high-temperature gas. The porous heat accumulating body is therefore exposed naturally to the severe heat shock environment, and also to a mechanical pressure caused by the passing gas under a thermal environment. Depending upon the kind of heat treatment carried out in the heating chamber, the porous heat accumulating body may be exposed to severe chemical reactions. Therefore, the porous heat accumulating body is required to be made of a stable material capable of bearing use in such a severe environment. Such a requirement does not however always assume presence of a heating chamber or a combustion burner, and is a technical requirement to be naturally satisfied when a regenerative heat exchanger is used in a severe environment. For example, when heat exchange is conducted through a porous heat accumulating body using a by-product gas from an industrial furnace as a high-temperature source, this applies if conceiving the industrial furnace itself as a “heating chamber” is not natural (a definition of the term “heating chamber” will be given in detail later).
The service life of a porous regenerator used in a heat treatment equipment is not endless. Since it is used under severe conditions, it is inevitable to periodically or non-periodically replace the porous regenerator. The period of replacement becomes shorter also for other reasons. For example, substances brought from outside the heat treatment equipment into the heating chamber (exogenous off-purpose substances) or substances brought into the heating chamber from members within the heat treatment equipment (including members themselves composing the porous regenerator) (endogenous off-purpose substances) are subjected to a heat treatment in the heating chamber, or chemically react with environmental substances in the heating chamber, or materials themselves of the porous regenerator react with other substances in the heating chamber, resulting in by-products, or these off-purpose substances do not fully change into by-products and remain in the heating chamber temporarily or for a long period of time in the non-reacting form. These by-products or residual off-purpose substances occur or deposit in the pores of the porous regenerator, causing choking or clogging of pores (hereinafter referred to as “choking” or the porous regenerator for convenience sake), resulting in deterioration of properties and quality of the porous regenerator with the lapse of time in service as compared with an originally expected level. As a result, a sudden increase in gas pressure is caused in the heating chamber, and combustion efficiency of a direct-fired burner, heat exchange efficiency of a regenerative heat exchanger, or performance as a whole also decreases, thus impairing efficient operation of the heat treatment equipment. In terms of the place for installation of the regenerative heat exchanger, if substances brought from outside into the place are considered exogenous substances, and substances coming from materials within the place as by-products are considered endogenous substances, the aforementioned phenomena illustrated as examples are directly applicable to a regenerative heat exchanger not requiring a heating chamber.
It thus becomes necessary to replace the porous regenerator. Unless choking of the porous regenerator is effectively prevented or inhibited, extending the service life is limited, so that the period for maintenance work such as inspection, maintenance, replacement, cleaning and the like of the porous regenerator cannot be increased to be over a certain level. This means that the frequency of these maintenance operations cannot be reduced to be under a certain level.
The easiest method for preventing or inhibiting choking of the porous regenerator is to conduct maintenance works more frequently such as periodical or non-periodical inspection, maintenance, replacement, cleaning and the like of the porous regenerator, placing too much importance on safety of the heat treatment equipment, assuming from the very beginning a limited service life of the porous regenerator, i.e., a life shorter than the actual one. It is true that frequent maintenance works of the porous regenerator would solve the problem of choking.
However, this causes an increase in the cost for maintenance works (including personnel cost and overhead). For example, a porous regenerator is unavailable without compensation. The increase in the replacement frequency of the porous regenerator is directly associated with the increase in maintenance and management cost of the heat treatment equipment. Even when maintenance is required only for some porous regenerators, it is sometimes necessary to discontinue operation of the heat treatment equipment. As a result, this leads to an increase in frequency of maintenance operations, causes troubles in operation of the heating equipment, and poses problems in operating cost.
Therefore, in order to solve a series of problems as described above caused by choking of the porous regenerator (hereinafter referred to as the “choking problem of porous regenerator” for convenience sake), it is necessary to contrive a technique capable of reducing the replacement frequency of the porous regenerator as far as possible. A porous regenerator achieved by such a technique should provide a higher cost curtailing effect than ever upon replacement after expiration of service life.
2. Progress of Choking and Substances Causing Choking
Off-purpose substances or heat treatment by-products (hereinafter generically referred to as “substances causing choking”) causing the choking problem of the porous regenerator are visible by naked eyes in some cases, or visible only through a microscope in some others. However, choking of the porous regenerator becomes more serious with the lapse of time, thus reducing the bore diameter of the pore. Presence and the degree of progress of choking can therefore be known from a change with time in the bore diameter of pore. Progress of choking causes a change in pressure or pressure loss of the gas flowing through the porous regenerator. Presence and the degree of progress of choking is therefore determinable through observation of this change. Irrespective of possibility to observe by naked eyes, presence and the degree of progress of choking can be known by setting an appropriate parameter. In the present invention, this parameter is comprehensively defined by use of a term “substantial average surface pore diameter”.
A typical example of exogenous off-purpose substances is dust comprising metals, ceramics, glass, oxides and the like present around the heat treatment equipment. Dust is hard-reactive in many cases. A substance, which, if not hard-reactive, produces detrimental by-products as a result of heat treatment, is as well included among exogenous off-purpose substances. Furthermore, substances contained in a fuel or combustion air for a direct-fired burner may also be considered to fall under the category of exogenous off-purpose substances. Particularly, when intentionally mixing powder or a solid, (for example, a solid fuel) with a fuel, and such powder or solid is not fully burnt but remains, the residue falling under this category. Some of such residues tend to promote generation of the above-mentioned by-products.
A typical example of endogenous off-purpose substances is a powdery or flaky residue produced as a result of wear or breakage of a porous regenerator through mechanical contact during service or an off-purpose substance produced as a result of application of a heat treatment to an object of treatment charged in a heating chamber. Examples of the latter, i.e., off-purpose substances coming from an object of treatment are oxides and other scale generated on the surface of the object of treatment as a result of a heat treatment in a heating chamber, comprising small particles separated from the object surface under the effect of mechanical pressure or heat shock of gases passing through the heating chamber or other environmental conditions (Japanese Unexamined Patent Application Publication No.7-119958). When arranging direct-fired burners or regenerative heat exchangers in an array shape, substances coming from component members of the porous regenerator (particularly, powdered substance described later) arranged in the former stage may be endogenous (exogenous in some configurations of the equipment) off-purpose substances for the porous regenerator arranged in the latter stage.
Off-purpose substance remaining in the heating chamber may physically clog off pores of the porous regenerator after the lapse of some time. Softening, melting or evaporation caused by heat in the heating chamber (chemical reactions in a sense) may promote clogging of pores of this porous regenerator. A low-temperature gas flows through the regenerator. Particularly when discontinuing equipment operation for maintenance operation, there occurs a decrease in temperature of the regenerator. In such a case, the off-purpose substance having once softened, melted or evaporated hardens, solidifies or condenses, clogging off the pores of the porous regenerator, thus causing the “choking problem”.
Comparison of a honey-comb type regenerator (defined later) and a ball type regenerator (defined later) reveals that, while in the former, the gas smoothly passes through straight pores, pores have non-straight portions in the latter, leading to a nonuniform gas flow rate which causes a serious decrease locally in the gas flow rate, resulting in easy stagnation or precipitation of off-purpose substances (particularly dust, and powdery or flaky residues). With at least the non-straight pores in view, however, the ball type regenerator tends to have a larger substantial average surface pore diameter as compared with the honey-comb type regenerator. Therefore, even if off-purpose substances tend to easily stagnate or precipitate at portions, the “choking” is not always more apparent in the ball type regenerator.
By-products are considered in many cases to result from chemical reactions at least between two substances from among exogenous or endogenous off-purpose substances, materials of the porous regenerator (for example, small amounts of compositions or impurities) and environmental substances in the heating chamber. These chemical reactions must have an important correlation with heat treatment conditions in the heating chamber (for example, contents and composition of the burner fuel, heat treatment temperature and heat treatment atmosphere). When a reaction generating by-products occurs in a porous regenerator, particularly on the surface thereof, the by-products gradually clog up pores of the porous regenerator. When there are exogenous or endogenous off-purpose substances in contact with the surface of the porous regenerator, for example, when there are off-purpose substances stagnating or precipitation as described above, by-products are slowly generated on the contact interface of the both, grow, and may accelerate choking of pores of the porous regenerator by off-purpose substances. Also, it is possible that heat treatment atmosphere may accelerate generation of by-products. This applies, for example, to a case where as substance of a low cleanliness such as a by-product gas of an industrial furnace including a blast furnace is used as fuel. When using a substance of a relatively high cleanliness such as a natural gas is used as fuel, unlike a by-product gas of an industrial furnace, “choking” caused by a by-product is not relatively serious. However, even when using a fuel material of a high cleanliness, “choking” may be caused by any other substance capable causing the same.
To judge from the definition of the above-mentioned endogenous off-purpose substance, i.e., “an off-purpose substance resulting from heat treatment applied to an object of treatment bought into the heating chamber”, it would be reasonable to consider a by-product to fall under the category of endogenous off-purpose substances. This member of reasoning provides an advantage of permitting collective explanation of substances causing “choking”, irrespective of what cleanliness a fuel material has. In the following description therefore, by-products are included in the category of endogenous off-purpose substance unless otherwise defined.
3. Conventional Art Solving “Choking Problem”
For the purpose of solving the problem of choking of porous regenerators, there have conventionally been available the following technologies:                (1) Dividing the regenerator into a plurality of layers, and forming the layer on the combustion gas inlet side so as to be replaceable from time to time (Japanese Unexamined Patent Application Publications Nos. 6-201276 and 8-94066).        (2) Providing a cyclone type dust collector on the waste gas inlet side of the regenerating chamber containing the regenerator and centrifugally separating scale and dust contained in waste gas (Japanese Unexamined Patent Application Publications Nos. 6-241420 and 7-119958).        (3) For reducing the concentration of NOx produced by a heat treatment, rinsing the regenerator by water or steam supplied through the regenerator into the heating chamber, and taking out substances tending to clog up pores without stopping the equipment or removing the regenerator for periodical or non-periodical maintenance work (Japanese Examined Patent Publication No. 4-70554).        (4) Providing a low-boiling-point metal catcher behind the waste gas outlet of a regenerator repeating passage of a combustion waste gas and combustion air in mutually reverse directions, and operating the heat treatment equipment so as to keep the waste gas temperature at regenerator exit at a level over the boiling point of the low-boiling-point metal so that condensation of the low-boiling-point occurs outside the regenerator (Japanese Unexamined Patent Application Publication No. 8-261421), or, providing a mist catcher in the middle of the waste gas channel connected to the regenerator (Japanese Unexamined Patent Application Publication No. 8-86419).        (5) In a regenerative continuous combustion burner of a type of causing continuous combustion of a single combustion burner by rotating a regenerator relative to the combustion air feed system and the exhausting system, providing reproducible high-melting-point and low-melting-point metal catching columns in the gas channel of the regenerator (Japanese Unexamined Patent Application Publication No. 8-86419).        (6) Dividing the heating chamber into upper and lower portions by means of a partition having a plurality of gaps to form a combustion chamber in which the burner actually produces flame and a melting chamber in which the object of treatment melts, and discharging dust or a low-boiling-point metal produced in the melting chamber through an auxiliary flue provided in the melting chamber located below the heating chamber to prevent inflow of these substances into the combustion chamber communicating with the regenerator (Japanese Unexamined Patent Application Publication No. 7-113579).        (7) Providing a regenerative alternate combustion burner at a position isolated from the flue to perform control so that a part of the combustion waste gas is discharged outside from the flue without passing through the regenerator to ensure that only the combustion waste gas mainly comprising a gas not containing a low-boiling-point metal passes through the regenerator (Japanese Unexamined Patent Application Publication No. 8-247430).        (8) Preventing production of (excessive) scale causing choking, by heat treating the object of treatment in an environment free from oxidation or with low oxygen (Japanese Unexamined Patent Application Publications Nos. 8-159664 and 7258740), or preventing the flame of the direct-fired burner from coming into contact with the object of treatment (Japanese Unexamined Patent Application Publication No. 7-102313).        
Apart from the above, the techniques of preventing rapid heating of the object of treatment and preventing cracks or bending of the object of treatment by providing a preheating chamber of the object of treatment in the heat treatment equipment (Japanese Unexamined Patent Application Publication No. 8-210780), in the sense of preventing granulation or dispersion of once produced scale, and the technique of reducing the quantity of produced scale by adjusting the gas pressure in the heating chamber (Japanese Unexamined Patent Application Publication No. 7-103461) can be a counter measure against the choking problem of porous regenerator.
Among other conventional technologies, the conventional art (1) is noteworthy as a proposal of an approach for solving the choking problem by making contrivances about the form of the porous regenerator itself to facilitate replacement of the regenerator surface layer in which choking has occurred, on the basis of findings that, in a direct-fired burner in which a combustion gas is introduced directly into a burner, endogenous substances such as scale of the high-temperature object of treatment or dust of pieces of refractory frequently comes in through pores of the porous regenerator, together with combustion waste gas, and adhere there, and such deposition largely occurs in the surface layer having a sufficient width on the combustion waste heat inlet side.
However, this does not relate to a technology for preventing or inhibiting the choking phenomenon itself. Particularly, this conventional art is a technology of transforming a surface layer of a porous regenerator guilt in a standalone regenerative burner or regenerative heat exchanger into a different shape or configuration, but has no relationship with a technical idea comprising contrivances about the form of a porous regenerator itself based on new findings about choking behavior in a plurality of regenerative burners or a plurality of regenerators in a regenerative heat exchanger.
From the point of view of a technology for preventing or inhibiting choking phenomenon itself, the conventional arts (2) to (8) are rather closer to the object than the conventional art (1). These conventional arts have, however, no relationship with a technology of preventing or inhibiting the choking phenomenon itself, paying attention to the form of the porous regenerator, and as in the case of the conventional art (1), have no relationship with a technical idea comprising contrivances about the form of a porous regenerator itself based on new findings about choking behavior in a plurality of regenerative burners or a plurality of regenerators in a regenerative heat exchanger.
The present invention was developed in view of a specific choking phenomenon in a plurality of regenerative burners or a plurality of porous regenerators in a regenerative heat exchanger installed in a heating chamber or a particular space in a heat treatment equipment, and has an object to solve the problem of choking of a porous regenerator by making contrivances about the form of the regenerator itself.